The disclosure relates to a projection exposure method for the exposure of a radiation-sensitive substrate arranged in the region of an image surface of a projection objective with at least one image of a pattern of a mask arranged in the region of an object surface of the projection objective, and to a projection exposure apparatus suitable for carrying out the method.
Microlithographic projection exposure methods are predominantly used nowadays for producing semiconductor components and other finely patterned components. These methods involve the use of masks (reticles), that bear the pattern of a structure to be imaged, e.g., a line pattern of a layer of a semiconductor component. A mask is positioned into a projection exposure apparatus between an illumination system and a projection objective in the region of the object surface of the projection objective and is illuminated with an illumination radiation provided by the illumination system. The radiation altered by the mask and the pattern passes as projection radiation through the projection objective, which images the pattern of the mask onto the substrate to be exposed, which normally bears a radiation-sensitive layer (photoresist).
There are various possibilities for transferring the image of a pattern of a mask to the substrate. In one variant, the entire pattern is positioned in the effective object field of the projection objective and imaged onto the substrate in an exposure extending over an exposure time interval, the mask and the substrate not moving during the exposure time interval. Corresponding projection exposure apparatuses are generally referred to as wafer steppers. In some systems, different regions of the pattern to be transferred are transferred to the substrate temporally successively. For this purpose, a scanning operation is carried out during an illumination time interval, said scanning operation involving the movement of the mask in the object surface relative to the effective object field of the projection objective, while the substrate is moved synchronously with the movement of the mask in the region of the image surface relative to the effective image field of the projection objective. The movement speed of the mask in the scanning direction thereof is linked to the movement speed of the substrate in the scanning direction thereof by the magnification ratio β of the projection objective, which is less than 1 in the case of reducing objectives. Projection exposure apparatuses designed for such scanning operations are generally referred to as wafer scanners.
In order that an image of the pattern that is as faithful to the original as possible is transferred to the substrate during the exposure process, the substrate surface should lie in the image-side focal range of the projection objective during the exposure time interval. In particular, the substrate surface should lie in the range of the depth of focus (DOF) of the projection objective, which is proportional to the Rayleigh unit RU, which is defined as RU=λ/NA2, the λ is the operating wavelength of the projection exposure apparatus and NA is the image-side numerical aperture of the projection objective. Lithography in the deep ultraviolet (DUV) range at λ=193 nm commonly utilizes, for example, projection objectives having image-side numerical apertures of 0.75 or greater in order to produce structure elements having typical sizes of 0.2 μm or less. In this NA range, the depth of focus is typically a few tenths of a micrometer. In general, the depth of focus becomes smaller, the higher the resolving power of the projection objective.
Lithography processes with a relatively large depth of focus are used for some applications. Large depths of focus are desired for example in the production of patterned semiconductor components for logic applications in order to produce SRAM cells, random contact holes or contact holes through pitch, for example. Large depths of focus can also be advantageous in so-called double patterning methods. In a double patterning method (or double-exposure method), a substrate, for example a semiconductor wafer, is exposed twice in succession and the photoresist is then processed further. In a first exposure process, a normal structure having a suitable structure width is projected. For a second exposure process, a second mask is used, which has a different mask structure. In particular, the structures of the second mask can be displaced by half a period relative to the structures of the first mask. In the general case, the differences between the layouts of the two masks can be large, particularly in the case of more complex structures. A reduction of the structure sizes that can be obtained at the substrate can be achieved by double patterning.
One possibility for enlarging the effective depth of focus is so-called “focus drilling”. Focus drilling involves altering the relative positioning of the surface of the substrate with respect to the focus surface of the projection objective during the exposure time interval in such a way that image points in the effective image field are exposed with different focus positions of the image of the mask during the exposure time interval.
This can be achieved for example by altering the operating wavelength of the light source that is used for imaging, thus resulting in different focus positions at the image side of the projection objective. It is also possible to use a projection objective containing one or more adjustable optical elements, such that it is possible to alter the focal length of the projection objective for the focus drilling. The focus drilling can then be achieved by modulating the focal length of the projection objective during the exposure time interval. These possibilities of focus drilling can be used for wafer steppers and for wafer scanners.
In the case of wafer scanners it is also possible to achieve focus drilling by tilting the substrate during the exposure time interval relative to the projection objective in a tilting direction about a tilting axis in such a way that the substrate normal is inclined by a finite tilting angle with respect to the optical axis of the projection objective in the region of the image plane. Together with a scanning movement of the substrate that proceeds parallel to the tilting direction, this leads, at each image point, to the superimposition of a multiplicity of aerial images at different focus positions, whereby the depth of focus of the process is effectively enlarged. Although the enlargement of the depth of focus that is achieved in this way is generally accompanied by a certain reduction of contrast, overall a more favourable process window and hence added value for the user can be produced.